Metal Oxide Varistors (MOV) are semiconductor devices that are fabricated using technology from the ceramic capacitor industry. MOV's that are presently produced can fracture violently when abused by pulse currents that exceed the rating of the device. MOV's are fabricated by milling alumina ceramic into a granular powder which can be coated by a metal oxide. This powder which is composed of grains of ceramic is coated by metal oxide. Then, the coated powder is pressed into a disc shape and sintered. At that point metalization is applied to each of the two sides, leads are attached and a protective epoxy coating is applied. An outer guard zone may be provided on the disc. Over current protection of the MOV and electronic circuit is accomplished by external fusing to the MOV. There is however, no integral thermal protection for the MOV so as to prevent and or reduce the occurrence of violent fracture.
MOV products are used to provide transient voltage protection for electronic equipment. MOV's are used in across-the-power-line applications and internal to an electronic circuit. The purpose of MOVs is to provide greater reliability in field applications where transient voltages in excess of normal high line voltages are present. These transient voltages can be generated by electric power utilities shifting loads, lightning strikes or induced lightning, and turning on and off of heavy equipment in industrial environments. Transient voltages can also be produced by the charging and discharging of inductors and large capacitors in electronic equipment.
MOV's utilization in across-the-power-line applications are at risk to transient voltage spikes that can exceed the capability of the MOV to absorb energy. This causes rapid heating in the MOV which will eventually lead to thermo-mechanical stress of the varistor and also has the potential for thermal damage to surrounding devices. Further, there is the potential for out gassing of materials which can cause the MOV to fracture violently into several sections. Usually, the violent fracture is caused by thermal expansion. These fractures usually will also dislodge part of the protective epoxy coating surrounding the MOV. An MOV that fractures due to overheating as a result of a voltage spike will become an open circuit. Current flow will be still available to the application and personnel operating the application will be unaware that the MOV has failed thus creating a potentially dangerous situation for personnel who may come in direct contact with the circuit and unaware of the MOV failure.